Cryptomatte Pick isn't working. What have I forgotten?
for me its working pretty good. using this since blender beta.
few tips to find out..

1- try this sample file
in my case, it looks like this. 

steps i did to check


create objects 
assign materials 
switch to cycles 
turn on cryptomatte (as you have already done ) 
hit render 
use cryptomatte nodes in compositor, tested via object and material both.